There are a few options I can see. If it's completely "either @a or @default", or if you want all of @a, and fill in with the first few from @default if @a is short, then you could do this:

@list=(@a,@default)[0..$#default];
[download]

As an alternative, you could do:

@list=map {$_>$#a ? $default[$_] : $a[$_] } 0..$#default;
[download]

It's a lot easier to do default values if you use hashes, though.
--
Mike